In past decades, there have come into widespread use communications systems which employ earth-orbiting satellites to relay communications between earth-based stations ("satellite communications systems"). Now under development and in early stages of deployment are satellite communications systems which utilize broadband signaling and operate in the 11-14 GHz (Ku) band, the 20-30 GHz (Ka) band, and higher millimeter wave bands between about 30 and 70 GHz. (Such systems are hereafter referred to as MMW systems or as Ku, Ka or V band systems.) Many of these MMW systems employ low-earth-orbit (LEO) or mid-earth-orbit (MEO) satellites in constellations, to provide bi-directional, high-speed data links from and to customer premises equipment (CPE) located at various places. They may also use one or more geostationary satellites (GEO's) in combination with LEO or MEO satellites in some types and modes of communications. The Spaceway, Expressway, Cyberstar, and Teledesic systems are the most well-known of these new systems; but in fact there are more than twenty MMW satellite systems in various stages of development and implementation.
All of these MMW systems are global in that they facilitate communications with CPE locations at virtually all points on the surface of the earth. End-user applications for these networks include placing CPE at various types of customer locations, including, for example, large and small business locations, telephone company points of presence (POP), multi-tenant office and apartment buildings, public telephone installations, and individual residences. One significant requirement for direct access to the satellite system from these locations is that a CPE antenna must be provided and that antenna must be capable of tracking simultaneously at least two LEO or MEO satellites to maintain a connection between the user and the network. In many cases, the antenna also must be able to track a geostationary satellite, as well. Further, it is important that such CPE antennas be low-cost devices capable of being made in a high-volume production environment.
At the customer location, the CPE antennas for these new MMW systems must acquire and track multiple satellites. In typical operations, a first LEO satellite is tracked across the sky from its acquisition horizon in the South or North, until it nears the opposite horizon. At that time, a second LEO satellite is acquired as it rises from its acquisition horizon. For a short time, both satellites are tracked, until the CPE link is handed off from the first satellite to the second satellite. This process is repeated as the various LEO or MEO satellites traverse the sky at each CPE location. In order to maintain operative communications links with the satellites of these MMW systems, the antennas used at the customer sites must be able to track at substantially any local azimuth angle and at all elevation angles above about fifteen (15) degrees. (Operation at lower elevation angles is not practical due to the increased atmospheric path loss and the presence of trees and buildings or other nearby structures.) This requires an antenna system with a full sky positioner mechanism that has sufficient accuracy to maintain tracking of a satellite such that a narrow, "pencil" transmission beam stays centered on the satellite as it moves across the sky. To assure adequate noise margin in the communications links, the CPE antennas are required to have diameters ranging from about 0.4 meters for "low end" (i.e., least expensive) residential units to "typical" diameters of about 0.75, 1.0 and 1.5 m for business CPE units. At these diameters, the beam widths at Ku, Ka band and V band are fractions of a degree; therefore, the positioner mechanism for the antenna must be capable of pointing the antenna to better than 0.1 degree of the target satellite position, at all points in the sky. This requires relatively precise positioners, and they often must move not only the entire MMW antenna but also its associated transmitter and receiver electronics.
Both electrically scanned and mechanically scanned antennas previously have been built for operation in satellite communication systems at the lower Ku and C band satellite communication (SATCOM) frequencies. In particular, the industry has a long history of providing Very Small Aperture Terminals (VSAT) for use in C and Ku band satellite links from customer premises. Such existing VSAT terminals use fixed mounted antennas that typically point to one specific GEO satellite location and do not need to be scanned or moved during operation. This fixed VSAT antenna approach is much easier to implement than the approach needed for the new MMW systems that must constantly track moving satellites across the sky.
Traditional parabolic reflectors, although easily manufactured in the small sizes needed, cannot be scanned readily with electronic means. Instead, they must be moved physically to point directly towards the orbiting satellite as it moves across the sky. In these MMW systems, this requires that the parabolic antenna be capable of being pointed to nearly all points in the entire hemisphere above some minimum elevation angle with respect to the horizon. Using traditional parabolic reflector antennas as are now used in Ku and C band VSAT antenna systems, in order to have multiple beams (to communicate simultaneously with multiple satellites), a separate reflector antenna is required for each beam, each having its own mechanical positioner mechanism. Multiple antennas must be used to facilitate the simultaneous tracking of two MEO or LEO satellites that are typically at opposite directions in the sky, and in the case where a GEO satellite is also employed, as many as three separate full-motion antennas must be employed simultaneously. Most of these antennas will be enclosed by a radome, both for aesthetic reasons and to avoid the effects of the environment (e.g., wind, ice, snow, insulation, etc.) on the tracking accuracy of the antenna system.
At the installation site for two (or three) relatively large and complex parabolic reflector tracking antennas, these antennas would be required to be spaced by about 5 m from each other so as not to interfere with each other during concurrent operation. Each antenna would be required to employ a relatively expensive and complicated mechanism to move the entire antenna to point towards the location of each tracked satellite as it moves in the sky. This relatively large installation will present an aesthetic and logistical problem at many locations. Since each antenna not only is moving constantly but also is transmitting RF energy, provisions for radomes must be made to avoid wind deflections and to make the installation safe from and for children, pets, etc.--particularly in residential and business sites. It is also not clear that a suitable high-accuracy positioner can be manufactured at a reasonable, low cost.
One might think of electrically scanned antennas as an alternative, but at the large scan angles required (i.e., nearly a full hemisphere coverage) in these new MMW systems, electrically scanned solid state phased arrays become difficult to implement. Moreover, they would have to have hundreds or even thousands of individual elements, making them both difficult to manufacture and quite costly. It is, however, technically feasible that monolithic GaAs semiconductor integrated circuits could provide 20-50-mW per transmit/receive element; and when used in groupings of around one hundred elements in a small array, this could provide 2-5 W of radiated power. However, the cost is an obstacle. The individual transmit/receive elements used in such solid state arrays at 20-30 GHz currently cost on the order of one hundred dollars each in modest production quantities. Therefore, it appears that arrays made from hundreds of such elements are not going to be affordable for the commercial market and residential markets in a near time frame; and possibly never will be affordable for the larger aperture sizes used for business terminals.
To achieve efficient beam scanning over very wide angular ranges, the fixed spherical reflector with a moveable feed has been known to offer a potentially attractive, low-cost alternative to a scanned parabolic reflector. With such a design, the primary reflector, which is the heaviest component of the system, remains fixed; beam scanning is effected by movement of a small and lightweight feed using a compact scanner mechanism. In its simplest form, a scanning spherical reflector consists of a fixed spherical reflector and a small scanning feed which moves along a spherical pseudo-focal surface located midway between the sphere center and spherical reflector surface. However, to achieve high efficiencies requires non-point source feed systems that use lenses or additionally shaped reflectors to correct for the spherical aberration of the main spherical reflector at the location of the point source feed.
Turning to FIG. 1, there is depicted in general the focus and pointing geometry of a spherical antenna as heretofore known. The aperture 12 of a spherically shaped reflector 10 collects radiation from a direction (.phi., .theta.) defined by the azimuth angle .phi. and the elevation angle .theta. with respect to the zenith direction. The incident radiation field 14 is a plane wave with its transverse electric and transverse magnetic field components in the plane perpendicular to, and its Poynting vector along the direction defined by (.phi., .theta.) in the region of the antenna aperture. Reflector 10 is a hemispherical surface. It collects electromagnetic energy from far field radiation sources, such that each signal arriving at the plane of the hemisphere's aperture 12 is a plane TEM wave that intersects the aperture plane at some angle (.phi., .theta.) relative to the major axis of the sphere.
Still referring to FIG. 1, for each plane TEM wave intersecting the hemisphere there is a corresponding location where the primary reflector 10 will produce a multitude of focal points 16 that extend in a line from the center point 18 of the radius of the sphere along the direction of the Poynting vector of each incident plane TEM wave as it cuts the plane of the spherical reflector. As a result, electromagnetic radiation 14 arriving at the aperture 12 from a far field source at angle (.phi., .theta.) is collected and focused along a focal line running along the direction (.phi., .theta.). The focal line direction passes through the center 20 of the sphere depending upon the direction to the far field radiation source. In many spherical antenna systems, a device called a "line feed" (not shown) is used to collect all the radiation appearing along the focal line region 16 from the far field source. Unfortunately, such line feeds are difficult to construct and do not usually have large instantaneous bandwidths. Furthermore, in the case of the MMW satellite systems where circular polarization is needed simultaneously at two widely spaced frequencies (one for transmitting and the other for receiving), it is doubtful that a practical low-cost line feed can be implemented which meets all the technical objectives.
Therefore, new approaches are needed for implementing cost-effective scanned antennas for MMW systems.